An underground steel pipe, which contains iron, is surrounded by damp soil containing air. A magnesium block attached to the pipe has corroded away completely.
Why is the pipe no longer protected from rusting?
The magnesium was a coating that kept water off the iron, and it has worn away.
The damp soil has made the iron more reactive than magnesium, so the protection stopped.
The magnesium made the iron less reactive, and this effect stopped when it was used up.
No magnesium is left to react with oxygen and water in place of the iron.
136 exam-style questions on OCR GCSE Chemistry 6.1 Improving processes and products, covering 6.1.1 Extracting metals industrially and by electrolysis, 6.1.2 Life cycle assessment and recycling, 6.1.3 Biological methods of metal extraction, 6.1.4 The Haber process and the production of fertilisers, 6.1.5 Alloys and their uses, 6.1.6 Corrosion and its prevention, 6.1.7 Comparing and selecting materials, and 6.1.8 Industrial conditions, rate and position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.